Piston

ABSTRACT

A piston for a free piston engine generator, comprising one or more elements arranged coaxially along a piston shaft wherein the length of the piston is at least five times its maximum diameter, wherein at least one of the elements is formed from a magnetically permeable composite material having isotropic electrical resistivity at least twice that of electrical steel this arrangement providing improved free piston position control, more consistent combustion and improved electrical conversion efficiency.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage of international Application PCT/GB2012/051850, filed Jul. 31, 2012, which international application was published on Feb. 14, 2013, as International Publication No. WO02013/021171. The International Application claims priority to British Patent Application No. 1117346.0. filed Aug. 10, 2011, the contents of which are incorporated herein by reference in their entireties.

The present invention relates to a piston and in particular a piston for a free piston engine generator.

In standard combustion engines, pistons are mechanically restrained within their cylinder as a result of being connected to a crankshaft, which is driven rotationally as a result of the reciprocating linear movement of the piston within the cylinder. In a free piston engine, however, the piston is not connected to a crankshaft, although pistons may be provided within an engine of this type that do have external mechanical linkages such as taught in U.S. Pat. No. 7,383,796.

Furthermore, it is known that electrical power can be generated by movement of a reciprocating piston in a free piston engine through one or more electrical coils to generate a magnetic flux change, for example U.S. Pat. No. 7,318,506. In this arrangement the piston carries a first coil and as it reciprocates within the cylinder it generates an electric current in a second coil that surrounds the cylinder. However, the piston is constructed from a solid piece of material that is permeable to magnetic flux and is necessarily very short relative to the length of the cylinder so that it may induce the flux changes as it passes through the second coil.

In existing free piston engines, the length of the piston is typically less than five times the diameter of the cylinder bore of the combustion chamber. The power output of the electrical machine in a free piston engine is determined by the area of the air gap, and to achieve an air gap area sufficient for a given combustion chamber geometry, which is determined by the diameter and swept volume, the diameter of the electrical machine is generally larger than the diameter of the combustion chamber. This change in diameter necessitates complex and expensive mechanical solutions to seal each combustion chamber, and to ensure that these are coaxially aligned with each other and with the axis of the intervening electrical machine.

Three fundamental design challenges for free piston engine generators are:

-   -   firstly to achieve high electrical efficiency in an electrical         machine, sized appropriately for the combustion power output of         the engine     -   secondly to achieve a good degree of piston motion control and         combustion chamber design so that the combustion process         efficiency and completeness can be assured     -   Thirdly, to resolve the first two challenges at the lowest         possible cost

The present invention provides an advantageous combination of features concerning the design of the piston, combustion chamber and electrical machine which together provide a highly efficient combustion process, allow highly efficient electrical power conversion of the combustion power output, provide a high electrical machine control force acting on low piston mass for more consistent combustion process control, and avoids the use of costly rare earth magnetic materials in the piston and complex design interfaces between combustion and electrical power generation mechanisms in the engine. The present invention therefore addresses each of the identified design challenges.

According to the present invention there is provided a piston for an engine generator, comprising one or more magnetically permeable elements, having isotropic magnetic permeability and electrical resistivity, arranged along a piston shaft and secured by clamping or other means such that contact is maintained between neighboring elements, wherein the length of the piston is at least five times its maximum diameter.

This ratio of piston length to piston diameter provides a better match between the power output of the combustion chamber and the power capacity of the air gap having an area equal to the cylindrical surface of the elongated piston. As a result, the air gap and combustion chamber diameters can be equivalent and no change in diameter is required between the combustion chambers at opposite ends of the piston. As a result, this piston enables a free piston engine to be constructed at lower cost than existing types of free piston engine.

Furthermore, the present invention provides a piston that is particularly effective in an engine generator, having a plurality of coils spaced along a cylinder in which the piston reciprocates, and whose coils provide an axial flux electrical machine topology in which toroidal magnetic flux circuits are coaxial with the direction of travel of the piston. The isotropic magnetic permeability and electrical resistivity of the piston elements permits this axial flux topology with minimal eddy current losses in the piston, offering higher efficiency and control force than other Free Piston Engine electrical machine concepts. Furthermore, the moving piston mass in this axial flux configuration is lower than is possible in equivalent transverse flux configurations, improving the control authority of the electromagnetic force exerted by the coils, and allowing improved piston motion control compared to transverse flux configurations.

Preferably, a piston crown is provided at each extremity of the piston to protect the core and spacer elements from the effects of combustion. Alternatively each piston may have a single piston crown facing the combustion chamber, the opposite end of the piston facing a bounce chamber that serves to reverse the direction of the piston at the end of each power stroke. Alternatively each piston may have a single piston crown facing a central combustion chamber between two opposed pistons moving in counter-phase with each other to achieve a fully balanced engine unit.

Preferably, the piston crown is constructed from a lightweight, temperature resistant and insulating material such as ceramic or titanium, and/or has a concave surface to reduce heat loss at top dead centre. Preferably the piston crown is coated with or constructed from a oxidation catalyst material that improves the consistency and completeness of combustion in the combustion chamber. Preferably the exposed surface area of the piston crown is at least twice the section area of the piston, increasing the efficacy of the combustion catalyst.

Preferably the piston shaft is hollow to reduce the piston mass, and the magnetically permeable element or elements coaxially arranged on the piston shaft are formed as annular rings having the same outer diameters. Preferably, part of the circumferential surface of the cylinder is coated in a friction reducing and wear-resistant material.

An example of the present invention will now be described with reference to the following figures, in which:

FIG. 1 shows a longitudinal section through a free piston engine generator cylinder having a piston according to an example of the present invention, the piston having a piston crown at both ends;

FIG. 2 is a longitudinal section through the piston of FIG. 1, showing the construction from elements coaxially arranged on a hollow piston shaft;

FIG. 3 is a perpendicular section through the piston, showing the concentric arrangement of the shaft and circular elements;

FIG. 4 is a longitudinal section through an alternative embodiment of the piston, having a piston crown at one end only;

FIG. 5 is a sectional view of the piston and cylinder of FIG. 1 illustrating the magnetic flux in switched stator elements caused by the coils and by movement of the piston according to the present invention;

FIG. 6 is a perpendicular section through a piston and cylinder showing the linear generator stator and magnetically permeable element in the piston arranged in close proximity;

FIG. 7 is a more detailed longitudinal section of the intake valve, intake sliding port valve apertures and fuel injector arrangement during the intake charge displacement scavenging phase;

FIG. 8 is a more detailed longitudinal section of the exhaust means including the exhaust valve and actuator during the exhaust phase;

FIG. 9 is a time-displacement plot showing the changing piston position within a cylinder during a complete engine cycle, and the timing of engine cycle events during this period;

FIG. 10 is a pressure-volume plot showing a typical cylinder pressure plot during a complete engine cycle;

FIG. 11 is a schematic longitudinal section through a cylinder at top dead centre, at the end of the compression phase and around the time of spark ignition and initiation of the combustion event in the first chamber;

FIG. 12 is a schematic longitudinal section through a cylinder mid way through the expansion phase of the first chamber;

FIG. 13 is a schematic longitudinal section through a cylinder at the end of the expansion phase, but before the intake and exhaust valves have opened;

FIG. 14 is a schematic longitudinal section through a cylinder following the opening of the intake valve to charge chamber 1, allowing intake charge fluid pressure to equalize the lower cylinder pressure in the first chamber;

FIG. 15 is a schematic longitudinal section through a cylinder following the opening of the exhaust valve, and whilst the intake valve remains open, scavenging the first chamber;

FIG. 16 is a schematic longitudinal section through a cylinder during fuel injection into the first chamber after the intake valve has closed;

FIG. 17 is a schematic longitudinal section through a cylinder during lubricant and/or coolant application onto the piston outer surface;

FIG. 18 is a schematic longitudinal section through a cylinder whilst the exhaust valve is open, and after the intake valve and sliding port valve have closed such that continuing expulsion of exhaust gases from the first chamber is achieved by piston displacement;

FIG. 19 is a schematic longitudinal section through a cylinder mid way through the compression phase in the first chamber;

In these figures and specification, the following labels are used:

1—Cylinder

1 a—First intake port aperture

1 b—Second intake port aperture

2—Piston

2 a—Piston outer surface

2 b—Piston end surface

2 c—Piston shaft

2 d—Piston crown

2 e—Lubrication control feature

2 f—Magnetically permeable piston core element

2 g—Non-permeable piston spacer element

2 h—Piston shaft end

2 i—Piston shaft cap

3—First combustion chamber

3 a—First combustion chamber height

3 b—First combustion chamber diameter

4—Second combustion chamber

5—Fuel injection means

5 a—Fuel

6—Intake means

6 a—Intake sliding port valve apertures

6 b—Air intake

6 c—Intake valve

6 d—Intake valve actuator

6 e—Intake charge compressor

6 f—Intake manifold

6 g—Intake valve recess

6 h—Intake channel

7—Exhaust means

7 a—Cylinder head

7 b—Exhaust valve

7 c—Exhaust valve actuator

7 d—Exhaust manifold channel

8—Ignition means

9—Linear generator means

9 a—Coils

9 b—Switching device

9 c—Magnetically permeable stator elements

9 d—Control module

9 e—Electrical output means

10—Lubricant and coolant application means

T1, T2, T3 & T4—Toroidal flux paths in the stator and piston elements

FIG. 1 shows an example of the present invention provided within a cylinder of a free piston engine electrical power generation system. It can be seen that the piston 2 is free to move along the length of the cylinder 1, the piston being constrained in coaxial alignment with the cylinder 1, thereby effectively partitioning the cylinder 1 into a first combustion chamber 3 and a second combustion chamber 4, each chamber having a variable volume depending on the position of the piston 2 within the cylinder 1. No part of the piston 2 extends outside the cylinder 1. Using the first chamber 3 as an example, each of the chambers 3, 4 has a variable height 3 a and a fixed diameter 3 b.

The cylinder 1 is, preferably, rotationally symmetric about its axis and is symmetrical about a central plane perpendicular to its axis. Although other geometric shapes could potentially be used to perform the invention, for example having square or rectangular section pistons, the arrangement having circular section pistons is preferred. The cylinder 1 has a series of apertures 1 a, 1 b provided along its length and distal from the ends, preferably in a central location. Through motion of the piston 2, the apertures 1 a, 1 b which are aligned with corresponding apertures 6 a, form a sliding port intake valve, which is arranged to operate in conjunction with an air intake 6 b provided around at least a portion of the cylinder 1, as is described in detail below.

FIG. 2 shows a piston 2 having an outer surface 2 a and comprising a central shaft 2 c onto which are mounted a series of cylindrical elements. These cylindrical elements may include a piston crown 2 d at each end of the central shaft 2 c, each piston crown 2 d preferably constructed from a temperature resistant and insulating material such as ceramic. The piston crown end surface 2 b is, preferably, slightly concave, reducing the surface area-to-volume ratios of the first and second chambers 3, 4 at top dead centre and thereby reducing heat losses. Alternatively, the piston crown 2 d may be formed from or coated with a material that acts as an oxidation catalyst to ensure the completeness of combustion in which case it is preferable that the exposed end surface of the piston crown 2 b has an area that is substantially larger than the section area of the piston, so that the catalyst surface action on the combustion chamber volume contents is enhanced. Of course, if the cylinder was of a different geometry then the configuration of these elements would be adapted accordingly.

Part of the piston outer surface 2 a may be coated in a friction reducing and wear-resistant material. The piston crown 2 d may include lubrication control features 2 e to control the degree of lubrication wetting of the cylinder 1 during operation of the engine. These lubrication control features may comprise a groove and an oil control ring as are commonly employed in conventional internal combustion engines.

One or more magnetically permeable core elements 2 f are mounted on the piston shaft 2 c. Each core element 2 f is constructed from a magnetically permeable material having isotropic magnetic permeability and isotropic electrical resistivity to reduce eddy current losses during operation of the engine.

Spacer elements 2 g are also mounted on the piston shaft 2 c. Each spacer element 2 g ideally has low magnetic permeability and is preferably constructed from a lightweight material such as aluminium alloy.

Preferably the magnetically permeable core elements are formed from an electrically permeable composite material such as Soft Magnetic Composite (SMC) having an isotropic electrical resistivity of greater than twice than of electrical steel and greater than 5.0×10⁻⁶ Ω·m in all directions. This isotropic characteristic permits the use of non-planar magnetic flux circuits such as those shown in FIG. 5 and described below, without these flux circuits causing high iron losses as a result of induced eddy currents. Such eddy currents would cause significant electrical losses this non-planar magnetic circuit topology was applied using conventional steel laminations since electrical resistivity in the plane of the laminations is typically very low.

The design of the magnetically permeable core elements 2 f and non-permeable spacer elements 2 g positions core elements 2 f at the correct pitch for efficient operation as, for example, part of a linear switched reluctance or switched flux electrical generator machine comprising the moving piston 2 and a linear generator means, for example a plurality of coils spaced along the length of the cylinder within which the piston reciprocates.

The total length of the piston is, preferably, at least five times its diameter and in any case it is at least sufficiently long to completely close the sliding port valve such that at no time does the intake channel 6 h allow combustion chambers 3 and 4 to communicate.

The piston shaft ends 2 h are mechanically deformed or otherwise fixed to the piston crowns 2 d such that the elements 2 f and 2 g that are mounted to the piston shaft 2 c are securely retained under the action of tension maintained in the piston shaft 2 c.

FIG. 3 is a sectional view of the piston 2, showing the piston shaft 2 c passing through a core element 2 f.

FIG. 4 shows an alternative embodiment of the present invention in which the piston 2 has a piston crown 2 d at one end only, the other end being sealed with a lightweight piston shaft cap element 2 i. This embodiment is suitable for use in an opposed piston free piston engine, or a free piston engine in which each cylinder has a single combustion chamber at one end, the other end having a bounce chamber that serves to reverse the direction of the piston at the end of each power stroke.

FIG. 5 shows an example of linear generator means 9 provided around the outside of the cylinder 1 and along at least a portion of its length, for facilitating the transfer of energy between piston 2 and electrical output means 9 e and for applying a force to control the position and movement of the piston. The linear generator means 9 includes a number of coils 9 a and a number of stators 9 c along the length of the linear generator means 9.

The linear generator means 9 may be of a number of different electrical machine types, for example a linear switched reluctance generator or a linear switched flux generator machine. In the arrangement shown, coils 9 a are switched by switching device 9 b so as to induce magnetic fields within stators 9 c and the piston core elements 2 f. In this embodiment, switching device 9 b varies the current in coils 9 a with a frequency of at least 100 Hz. This switching is precisely timed in relation to the movement of the piston so that the piston's movement cuts the flux generated by the current in the coils, applying a force on the piston and transferring kinetic energy from the piston into electrical power in the coils.

In the arrangement shown in FIG. 5, toroidal magnetic flux circuits T1-T4 are created in the stators 9 c and piston core elements 2 f under the action of the switched current in coils 9 a. The linear generator means 9 functions as a linear switched reluctance device, or as a linear switched flux device. Power is generated at the electrical output means 9 e as a consequence of the magnetic flux circuits T1-T4 being cut by the motion of the piston 2 and thereby inducing current in coils 9 a. The toroidal topology of the magnetic flux circuits provides an exceptionally high flux density and rate of flux cutting per unit mass of magnetically permeable elements 2 f, thereby increasing the control force acting on each unit of mass of the piston and so improving the control authority of the electrical machine over the position and movement of the piston. In addition, this arrangement permits a highly efficient electrical generation means without the use of permanent magnets in the piston, which may demagnetise under the high temperature conditions within an internal combustion engine, and which might otherwise add significant cost to the engine due the use of costly rare earth metals.

Additionally, a control module 9 d may be employed, comprising several different control means, as described below. The different control means are provided to achieve the desired rate of transfer of energy between the piston 2 and electrical output means 9 e in order to deliver a maximum electrical output whilst satisfying the desired motion characteristics of the piston 2, including compression rate and ratio, expansion rate and ratio, and piston dwell time at top dead centre of each chamber 3, 4.

A valve control means may be used to control the intake valve 6 c and the exhaust valve 7 b. By controlling the closure of the exhaust valve 7 b, the valve control means is able to control the start of the compression phase. In a similar way, the valve control means can also be used to control exhaust gas recirculation (EGR), intake charge and compression ratio.

A compression ratio control means that is appropriate to the type of electrical machine may also be employed. For example, in the case of a switched reluctance machine, compression ratio control is partially achieved by varying the phase, frequency and current applied to the switched coils 9 a. This changes the rate at which induced transverse flux is cut by the motion of the piston 2, and therefore changes the force that is applied to the piston 2. Accordingly, the coils 9 a may be used to control the kinetic energy of the piston 2, both at the point of exhaust valve 7 b closure and during the subsequent deceleration of the piston 2.

A spark ignition timing control means may then be employed to respond to any residual cycle-to-cycle variability in the compression ratio to ensure that the adverse impact of this residual variability on engine emissions and efficiency are minimized, as follows. Generally, the expected compression ratio at the end of each compression phase is the target compression ratio plus an error that is related to system variability, such as the combustion event that occurred in the opposite combustion chamber 3, 4, and the control system characteristics. The spark ignition timing control means may adjust the timing of the spark ignition event in response to the measured speed and acceleration of the approaching piston 2 to optimize the combustion event for the expected compression ratio at the end of each compression phase.

The target compression ratio will normally be a constant depending on the fuel 5 a that is used. However, a compression ratio error may be derived from any variation of the final combustion chamber height 3 a at top dead centre. Hence if a chamber height variation of +/−20% arises, and the target compression ratio is 12:1, the actual compression ratio may be in the range 10:1 to 15:1. Advancement or retardation of the spark ignition event by the spark ignition timing control means will therefore reduce the adverse emissions and efficiency impact of this error.

Additionally, a fuel injection control means may be employed to control the timing of the injection of fuel 5 a so that it is injected into a combustion chamber 3, 4 immediately prior to the sliding port valve aperture 6 a closing to reduce hydrocarbon (HC) emissions during scavenging.

Furthermore, a temperature control means may be provided, including one or more temperature sensors positioned in proximity to the coils 9 a, electronic devices and other elements sensitive to high temperatures, to control the flow of cooling fluid applied by coolant application means 10, and the flow of cooling air provided by the compressor 6 e in response to detected temperature changes. The temperature control means may be in communication with the valve control means to limit engine power output when sustained elevated temperature readings are detected to avoid engine damage.

Further sensors that may be employed by the control module 9 d preferably include an exhaust gas (Lambda) sensor and an air flow sensor to determine the amount of fuel 5 a to be injected into a chamber according to the quantity of air added, for a given fuel type. Accordingly, a fuel sensor may also be employed to determine the type of fuel being used.

FIG. 6 shows a perpendicular section through one of the stator elements 9 c, showing the close proximity of the stator elements 9 c and the magnetically permeable piston elements 2 f, separated by the wall thickness of cylinder 1. In addition, FIG. 6 show the coaxial arrangement of magnetically permeable elements 2 f with the hollow piston shaft 2 c.

FIG. 7 shows the intake means 6 provided around the cylinder 1, the intake means 6 comprising sliding port valve apertures 6 a, which are a corresponding size and align with the apertures 1 a, 1 b provided in the cylinder 1, and an air intake 6 b. The apertures 6 a in the intake means 6 are connected by a channel 6 h in which an intake valve 6 c is seated. The channel 6 h is of minimal volume, either having a short length, small cross sectional area or a combination of both, to minimize uncontrolled expansion losses within the channel 6 h during the expansion phase.

The intake valve 6 c seals the channel 6 h from an intake manifold 6 f provided adjacent to the cylinder 1 as part of the air intake 6 b. The intake valve 6 c is operated by a intake valve actuator 6 d, which may be an electrically operated solenoid means or other suitable electrical or mechanical means.

When the sliding port intake valve aperture 6 a and the intake valve 6 c are both open with respect to one of the first or second chambers 3, 4, the intake manifold 6 f is in fluid communication with that chamber via the channel 6 h. The intake means 6 is preferably provided with a recess 6 g arranged to receive the intake valve 6 c when fully open to ensure that fluid can flow freely through the channel 6 h.

The air intake 6 b also includes an intake charge compressor 6 e which may be operated electrically, mechanically, or under the action of pressure waves originating from the air intake 6 b. The intake charge compressor 6 e can also be operated under the action of pressure or pressure waves originating from an exhaust means 7 provided at each end of the cylinder 1, as described below. The intake charge compressor 6 e may be a positive displacement device, centrifugal device, axial flow device, pressure wave device, or any suitable compression device. The intake charge compressor 6 e elevates pressure in the intake manifold 6 f such that when the air intake 6 b is opened, the pressure in the intake manifold 6 f is greater than the pressure in the chamber 3, 4 connected to the intake manifold 6 f, thereby permitting a flow of intake charge fluid.

Fuel injection means 5 are also provided within the intake means 6, such as a solenoid injector or piezo-injector 5. Although a centrally positioned single fuel injector 5 may be adequate, there is preferably a fuel injector 5 provided either side of the intake valve 6 c and arranged proximate to the extremities of the sliding port valve apertures 6 a. The fuel injectors 5 are preferably recessed in the intake means 6 such that the piston 2 may pass over and past the sliding port intake valve apertures 6 a and air intake 6 b without obstruction. The fuel injectors 5 are configured to inject fuel into the respective chambers 3, 4 through each of the sliding port intake valve apertures 6 a

Lubricant and coolant application means 10 are provided, preferably recessed within the intake means 6 and arranged such that the piston 2 may pass over and past the intake means 6 without obstruction, whereby the piston may be lubricated and cooled by the application of one or more fluids. The fluids applied may include conventional lubrication oil. In addition, the fluids may include water or another volatile liquid having a high heat of vaporisation. As this coolant fluid evaporates, heat in the piston is transferred into the fluid and resulting gas either vents in the combustion chamber as the piston travels, or resists the leakage of combustion blowby gases as the combustion chamber expands. The venting coolant gas increases the quantity of gas in the expanding combustion chamber and thereby incrementally increases the combustion chamber pressure. In this way, heat build up in the piston due to eddy currents caused by changing magnetic flux and by conduction from the combustion chambers may be recuperated as useful work applied to the piston by the expanding combustion chamber volume. In addition the venting gas may act as a gas bearing, reducing the friction and wear on the outer surface of the piston 2 a.

FIG. 8 shows the exhaust means 7 provided at each end of the cylinder 1. The exhaust means 7 comprises a cylinder head 7 a removably attached, by screw means or similar, to the end of the cylinder 1. Within each cylinder head 7 a is located an exhaust valve 7 b, coaxially aligned with the axis of the cylinder 1. The exhaust valve 7 b is operated by an exhaust valve actuator 7 c, which may be an electrically operated solenoid means or other electrical or mechanical means. Accordingly, when the intake valve 6 c and the exhaust valve 7 b within the first or second combustion chambers 3, 4, are both closed, that chamber is effectively sealed and a working fluid contained therein may be compressed or allowed to expand.

The exhaust means 7 also includes an exhaust manifold channel 7 d provided within the cylinder head, into which exhaust gases may flow, under the action of a pressure differential between the adjacent first or second chamber 3, 4 and the fluid within the exhaust manifold channel 7 d when the exhaust valve 7 b is open. The flow of the exhaust gases can be better seen in the arrangement of cylinders illustrated in FIG. 20, which shows the direction of the exhaust gas flow to be substantially perpendicular to the axis of the cylinder 1.

Ignition means 8, such as a spark plug, are also provided at each end of the cylinder 1, the ignition means 8 being located within the cylinder head 7 a and, preferably, recessed such that there is no obstruction of the piston 2 during the normal operating cycle of the engine.

The, preferably, coaxial arrangement of the exhaust valve 7 b with the axis of the cylinder 1 allows the exhaust valve 7 b diameter to be much larger relative to the diameter of the chambers 3, 4 than in a conventional internal combustion engine.

Each cylinder head 7 a is constructed from a hard-wearing and good insulating material, such as ceramic, to minimize heat rejection and avoid the need for separate valve seat components.

FIG. 9 shows a time-displacement plot of an engine according to the present invention, illustrating the movement of the piston 2 over the course of a complete engine cycle. Although the operation of the engine is described here with reference to the first chamber 3, a skilled person will recognize that the operation and sequence of events of the second chamber 4 is exactly the same as the first chamber 3, but 180 degrees out of phase. In other words, the piston 2 reaches top dead centre in the first chamber 3 at the same time as it reaches bottom dead centre in the second chamber 4.

The events A to F, highlighted throughout the engine cycle, correspond to the events A to F illustrated in FIG. 10, which shows a typical pressure-volume plot for a combustion chamber 3, 4 over the course of the same engine cycle. The events featured in FIGS. 9 to 10 are referred to in the following discussion of FIGS. 11 to 19.

Considering now a complete engine cycle, at the start of the engine cycle, the first chamber 3 contains a compressed mixture composed primarily of pre-mixed fuel and air, with a minority proportion of residual exhaust gases retained from the previous cycle. It is well known that the presence of a controlled quantity of exhaust gases is advantageous for the efficient operation of the engine, since this can reduce or eliminate the need for intake charge throttling as a means of engine power modulation, which is a significant source of losses in conventional spark ignition engines. In addition, formation of nitrous oxide pollutant gases are reduced since peak combustion temperatures and pressures are lower than in an engine without exhaust gas retention. This is a consequence of the exhaust gas fraction not contributing to the combustion reaction, and due to the high heat capacity of carbon dioxide and water in the retained gases.

FIG. 11 shows the position of the piston relative to the cylinder 1, defining the geometry of the first chamber 3 at top dead centre (A). This is also around the point of initiation of the combustion phase AB. The distance between the top of the piston 2 b and the end of the first chamber 3 is at least half the diameter of the first chamber 3, giving a lower surface area to volume ratio compared to combustion chambers in conventional internal combustion engines, and reducing the heat losses from the first chamber 3 during combustion. The ignition means 8 are recessed within the cylinder head 7 a so that in the event that the piston 2 approaches top dead centre in an uncontrolled manner there is no possibility of contact between the ignition means 8 and the piston crown 2 d. Instead, compression will continue until the motion of the piston 2 is arrested by the continuing build up of pressure due to approximately adiabatic compression in the first chamber 3. With reference to FIG. 10, the combustion expansion phase AB is initiated by an ignition event (A).

FIG. 12 shows the position of the piston 2 relative the linear generator means 9 mid-way through the expansion phase (AB and BC). The first chamber 3 expands as the piston 2 moves under the action of the pressure differential between the first chamber 3 and the second chamber 4. The pressure in the second chamber 4 at this point is approximately equivalent to the pressure in the intake manifold 6 f. The expansion of the first chamber 3 is opposed by the action of the linear generator means 9, which may be modulated in order to achieve a desired expansion rate, to meet the engine performance, efficiency and emissions objectives.

FIG. 13 shows the position of the piston 2 at bottom dead centre relative to the first chamber 3. At the end of the expansion phase (C), the motion of the piston 2 is arrested under the action of the linear generator means 9 and the pressure differential between the first chamber 3 and the second chamber 4. The pressure in the second chamber 4 at this point is approximately equal to the high pressure in the first chamber 3 at its top dead centre position (A). Preferably, the expansion ratio is at least two times the compression ratio, wherein the compression ratio is in the range of 10:1 to 16:1. This gives an improved thermal efficiency compared to conventional internal combustion engines wherein the expansion ratio is similar to the compression ratio.

FIG. 14 shows the arrangement of the piston 2 and intake means 6 and the initial flow of intake gas at the time of bottom dead centre during the intake equalization phase (CD). This arrangement can also be seen in FIG. 7. At this point, the sliding port intake valve 6 a is open due to the piston 2 sliding through and past the apertures 1 a, 1 b provided along the inner wall 1 c of the cylinder 1. The pressure in the first chamber 3 is lower than the pressure in the intake manifold 6 f due to the over-expansion reducing fluid pressure in the first chamber 3 and due to the intake compressor 6 e elevating the pressure in the intake manifold 6 h. Around this time, the intake valve 6 c is opened by intake valve actuator 6 d allowing intake charge to enter the first chamber 3 within cylinder 1 whose pressure approaches equalization with the pressure at the intake manifold 6 f. A short time after the intake valve 6 c opens, the exhaust valve 7 b is also opened allowing exhaust gases to exit the first chamber 3 under the action of the pressure differential between the first chamber 3 and the exhaust manifold channel 7 d, which remains close to ambient atmospheric pressure.

FIG. 15 shows the position of the piston 2 during the intake charge displacement scavenging phase (DE). Exhaust gas scavenging is achieved by the continuing displacement of exhaust gas in the first chamber 3 into the exhaust manifold channel 7 d with fresh intake charge introduced at the piston end of the first chamber 3. Once the intended quantity of intake charge has been admitted to the first chamber 3, the intake valve 6 c is closed and the expulsion of exhaust gas continues by the movement of the piston 2, as shown in FIG. 17, explained below.

FIG. 16 shows the arrangement of the piston 2 and intake means 6 at the point of fuel injection (E). If a liquid fuel is used, this fuel 5 a may be introduced directly onto the approaching piston crown 2 d which has the effects of rapidly vaporizing fuel, cooling the piston crown 2 d and minimizing the losses and emissions of unburned fuel as a wet film on the inner wall 1 c of the cylinder 1, which might otherwise vaporize in the second chamber 4 during the expansion phase.

FIG. 17 shows the position of the piston 2 during lubrication (E), whereby a small quantity of lubricant and/or coolant is periodically introduced by the lubricant and coolant application means 10 directly to the piston outer surface 2 a as it passes the intake sliding port valve 6 a. This arrangement minimizes hydrocarbon emissions associated with lubricant wetting of the cylinder inner wall, and may also reduce the extent of dissolution of fuel in the cylinder inner wall oil film. Lubrication control ring features 2 e are included in the piston crown 2 d to further reduce the extent of lubricant wall wetting in the first and second chambers 3, 4.

FIG. 18 shows the position of the piston 2 during the piston displacement scavenging phase EF. The intake valve 6 c is closed and the expulsion of exhaust gas continues by the movement of the piston 2. The piston 2 at this time is moving towards the exhaust means 7 and reducing the volume of the first chamber 3 due to the combustion event in the second chamber 4.

As a result of the larger diameter of the exhaust valve relative to the combustion chamber diameter 3 b, the limiting area in the exhaust flow past the valve stem may approach 40% of the cylinder bore section area, resulting in low exhaust back pressure losses during both the intake charge displacement scavenging phase (DE) and piston displacement scavenging phase (EF).

FIG. 19 shows a longitudinal section of the position of the piston 2 relative to the cylinder 1 mid-way through the compression phase (FA). When a sufficient exhaust gas expulsion has been achieved, such that the proportion of exhaust gas in the fluid in the first chamber 3 is close to the intended level, the exhaust valve 7 b is closed and the compression phase (FA) begins. Compression continues at a varying rate as the piston 2 accelerates and decelerates under the action of the pressure differential between the first chamber 3 and the second chamber 4 and by the action of the linear generator means 9. The pressure in the second chamber 4 is at this point falling during the expansion phases (AB and BC). The linear generator force may be modulated in order to achieve the desired compression rate to meet the engine performance, efficiency and emissions objectives. The volumetric compression rate in the first chamber 3 is substantially equal to and opposite the volumetric expansion rate in chamber 4. 

The invention claimed is:
 1. A piston for a free piston engine generator, comprising one or more elements arranged coaxially along a piston shaft wherein the length of the piston is at least five times its maximum diameter, wherein at least one of the elements is formed from a magnetically permeable material having isotropic electrical resistivity greater than twice that of electrical steel.
 2. The piston of claim 1 wherein the elements are formed from an electrically permeable composite material having an isotropic electrical resistivity of greater than 5.0×10⁻⁶ Ω·m in all directions.
 3. The piston of claim 1 arranged within a magnetic flux generating, means comprising a plurality of coils and stator elements, arranged in close proximity to the piston so that the movement of the piston causes variation in the magnetic flux applied by the magnetic flux generating means.
 4. The piston and magnetic flux generating means of claim 3, also having switching means by which the magnetic flux may be varied with a frequency of at least 100 Hz.
 5. The piston of claim 1 arranged within a free piston engine having a liquid spraying means that permits a coolant and/or lubricating liquid to be sprayed directly onto the piston surface.
 6. The piston of claim 1 wherein at least one of the elements is formed from a non-magnetising material and each element being secured such that contact is maintained between neighbouring elements.
 7. The piston of claim 1 further comprising a piston crown provided at one or both extremities of the piston.
 8. The piston of claim 7, wherein the piston crown is ceramic.
 9. The piston of claim 7, wherein the piston crown is concave.
 10. The piston of claim 7, wherein the exposed surface area of the piston crown is at least twice the section area of the piston, and whose exposed surface material acts as a catalyst to promote oxidation of combustion chamber contents.
 11. The piston of claim 7, wherein the piston shall is hollow.
 12. The piston of claim 1, wherein the elements coaxially arranged on the piston shall are formed as annular rings having the same outer diameters.
 13. The piston of claim 1, wherein part of the circumferential surface of the cylinder is coated in a friction reducing and wear-resistant material.
 14. A method of applying a force on the moving piston of claim 1 by inducing a plurality of toroidal magnetic flux circuits coaxial with the axis of the piston, using external electromagnetic means, such that the peak flux density in the piston is greater than the maximum residual flux density present when the externally generated toroidal magnetic flux circuits are absent.
 15. The method of claim 14 where the toroidal flux is controlled by a switching the electromagnetic flux generating means at a frequency of at least 100 Hz.
 16. The method of claim 14, wherein a liquid is sprayed onto the piston so that heat generated in the piston by the varying magnetic flux and by combustion processes is recuperated as the liquid evaporates, providing a gas bearing for the piston and increasing the engine efficiency. 